Compressively strained SiGe-on-insulator (SGOIs) or SiGe/Si-on-insulator materials which have been patterned into islands over the oxide have shown to relax by expansion of the SiGe layer on the viscous oxide during a high-temperature annealing step (T>1150° C.). This method of forming highly relaxed, high quality SGOI in conjunction with thermal mixing is described, for example, in U.S. Ser. No. 10/300,189, filed Nov. 20, 2002, which application was previously incorporated herein by reference.
The extent of relaxation is, in general, greater than would occur if the SiGe film was not patterned. Furthermore, the patterned and annealed SGOI islands have a significantly lower crystal defect density than continuous SiGe layers which have been relaxed.
Because patterning of initially continuous semiconductor films into islands is one of the first steps in modern silicon-on-insulator (SOI) complementary metal oxide semiconductor (CMOS) processing, it is natural to merge the relaxation step of the SGOI material with the patterning step (i.e., shallow trench isolation). This way, the material advantage of enhanced relaxation and defect reduction obtained by relaxing the patterned SiGe layers is streamlined by integrating the SGOI formation with an existing SOI CMOS processing step.
One of the challenges facing this approach is that the island features are well-defined by the CMOS technology and should not be altered or distorted by the high-temperature SiGe relaxation annealing step. If the sidewalls of the SiGe or SiGe/Si islands are oxidized during the relaxation anneal, then the consumption of the sidewall during this process will change the dimensions of the island.
Another related concern is that oxidation of a SiGe material tends to reject the Ge from the growing oxide layer. Therefore, the Ge fraction in smaller island sizes (where the surface/volume ratio is high) will increase more rapidly than the Ge content in larger islands (where sidewall oxidation effects are negligible with respect to the Ge content). Furthermore, annealing performed in an inert ambient grows some amount of oxide at these high temperatures due to the presence of trace amounts of moisture in the annealing gases.
In view of the above, there is a need for providing a way to limit and/or eliminate the oxidation of the sidewalls of the SiGe-containing islands in order to maintain (1) the original lateral dimensions of the patterned islands and (2) a uniform and homogeneous Ge fraction which is independent of island size while still permitting island relaxation to occur.